Using sleep to limit frames is not generally a good idea. At least that's what most people say here.
Use sleep so that your app doesn't hog resources.
With interpolation, you need to save the old positions of all your entities and interpolate between the old and the new positions based on normalized dt (0 to 1).
This article might be of use:http://games.freebasic.net/BASICGaming/Issue9/index.html#tutorial1
I think the issue is that since I reduced the number of UPS from 60 to 30 the game behaves differently, even IF I change the values.
well I have to sleep to not hog resourced, But when should I sleep?
I usually sleep till the next update or render ( whichever is sooner)
I modified my moveable interface into an abstract class and made it that if you set X/Y or do MoveX/MoveY it automatically saves the previous
as well as doing lerp and stuff
It works rather good, at least movement isn't worse than before but still not optimal, I will try to provide 2 versions, unfortunately I didn't use a version control system, so I don't have older code, stupid of me I know...